1. Field of the Invention
The present invention relates to a semi-conductive roll which is suitably used as a charging roll, for example, in an image forming apparatus such as an electrophotographic copying machine, printer or the like.
2. Discussion of the Related Art
A semi-conductive roll such as a charging roll or a developing roll is installed in an image forming apparatus such as an electrophotographic copying machine, printer or the like, such that the semi-conductive roll is held in rolling contact with a photosensitive drum. For instance, the charging roll is used in a roll charging method wherein a photosensitive drum on which an electrostatic latent image is formed is charged by the charging roll. Described more specifically, in the roll charging method, the charging roll and the photosensitive drum are rotated such that the charging roll to which a voltage is applied is held in pressing contact with an outer circumferential surface of the photosensitive drum, to thereby charge the outer circumferential surface of the photosensitive drum. The developing roll carries a toner on its outer circumferential surface. The photosensitive drum and the developing roll are rotated such that the developing roll is held in pressing contact with the outer circumferential surface of the photosensitive drum on which the latent image is formed, so that the toner is transferred from the developing roll onto the photosensitive drum, whereby the latent image is developed into a visible image.
Since the charging roll and the developing rolls as described above are held in rotating contact with the outer circumferential surface of the photosensitive drum, such rolls are required to exhibit low hardness or high flexibility, and a moderate degree of electrical conductivity for charging the photosensitive drum.
In recent years, there is an increasing demand for higher image reproducing capability and excellent energy-saving characteristic of the image forming apparatus, as well as higher process speed and excellent durability of the image forming apparatus. In an attempt to improve the energy-saving characteristic for reducing the electric power to be consumed by the image forming apparatus, the melting point of the toner is lowered for the purpose of fixing the toner on a recording medium at a lower temperature. Further, for improving the performance of the image forming apparatus to provide sufficiently high image quality, the size of the toner particles is made smaller.
However, when the melting point of the toner is lowered or the size of the toner particles is made smaller, the toner undesirably tends to adhere to the semi-conductive roll such as the charging roll and the developing roll as described above which are inevitably heated in the image forming apparatus during the operation of the apparatus. The toner which adheres or clings to the semi-conductive roll causes deterioration of the image reproducing capability of the image forming apparatus. Described more specifically, with an increase in the number of copying or printing operations, in other words, with an increase in the number of operations to develop the electrostatic latent images into visible toner images on the outer circumferential surface of the photosensitive drum, the electric resistance of the semi-conductive roll such as the charging roll tends to be raised as a whole due to the adhesion of the toner to the semi-conductive roll. Further, uneven adhesion of the toner to local portions of the roll causes a variation in the resistance of the roll at the local portions. As a result, the image to be reproduced by the image forming apparatus is undesirably deteriorated.
As one measure for preventing the deterioration of the copying or printing quality, it is effective to prevent the toner from adhering to the surface of the semi-conductive roll such as the charging roll. However, a conventional roll whose outermost layer is formed of a hydrophilic resin such as N-methoxymethylated nylon suffers from variation of the electric resistance under the operation at high temperature and high humidity. In addition, the conventional roll is not capable of effectively preventing the adhesion of the toner to its surface. In place of such a nylon resin, a fluoro resin is used for forming the outermost layer of the roll since the fluoro resin permits relatively easy removal of the outermost layer from a mold used for forming the roll. However, the adhesion of the toner to the roll surface is not prevented to a satisfactory extent even in the roll whose outermost layer is formed of the fluoro resin as described above. Namely, with the increase in the number of copying or printing operations, the toner is likely to adhere to the roll surface. Thus, the conventional semi-conductive rolls do not exhibit sufficiently high durability.